Wireless communication system, wireless communication apparatus, and wireless communication method

ABSTRACT

A wireless communication system is disclosed. The system performs data transmission from a first terminal including N antennas to a second terminal including M antennas using spatially multiplexed streams (N and M are integers larger than or equal to 2).

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/790,594, filed Apr. 26, 2007 now U.S. Pat. No. 7,848,444, which isbased on and claims the benefit of priority under 35 U.S.C. §119 fromJapanese Patent Application No. 2006-124537 filed in the Japanese PatentOffice on Apr. 27, 2006 and Japanese Patent Application No. 2007-056243filed in the Japanese Patent Office on Mar. 6, 2007. The entire contentsof the above-referenced applications are hereby incorporated byreference.

BACKGROUND

1. Technical Field

In one aspect, the invention relates to a wireless communication system,a wireless communication apparatus, and a wireless communication methodusing spatial multiplexing, and more particularly, to a wirelesscommunication system, a wireless communication apparatus, and a wirelesscommunication method, in which a transmitter and a receiver sharechannel information to perform closed loop type spatial multiplexingtransmission.

In another aspect, the invention relates to a wireless communicationsystem, a wireless communication apparatus, and a wireless communicationmethod, which perform beamforming by obtaining a channel matrix on thebasis of training series transmitted from a receiver when a transmittertransmits a packet, and more particularly, to a wireless communicationsystem, a wireless communication apparatus, and a wireless communicationmethod, which perform beamforming using the training series transmittedfrom the transmitter to the receiver when a number of antennas of thetransmitter, which is a beamformer, is smaller than that of thereceiver, which is a beamformee.

2. Background Art

Wireless networks have attracted attention recently. A standard ofwireless network may be IEEE (Institute of Electrical and ElectronicsEngineers) 802.11 or IEEE 802.15.

For example, IEEE 802.11a/g, a standard of wireless Local Area Network(LAN), employs an orthogonal frequency division multiplexing (OFDM)modulation method, which is a multi-carrier method. Because, in the OFDMmodulation method, transmission data having orthogonal frequencies isdistributed to a plurality of carriers and transmitted, the band of eachcarrier becomes narrow, spectrum efficiency is very high, and resistanceto frequency-selective fading interference is strong.

In addition, IEEE 802.11a/g standard supports a modulation method foraccomplishing a communication speed up to 54 Mbps. However, anext-generation wireless LAN standard requires a higher bit rate.

In order to realize a higher speed for wireless communications,multi-input multi-output (MIMO) communication has attracted attention.MIMO communication employs a plurality of antennas in a transmitter andin a receiver to realize spatially multiplexed streams. The transmitterperforms spatial/temporal encoding and multiplexing of plural pieces oftransmission data, and distributes and transmits the plural pieces oftransmission data to N transmission antennas through channels, where Nis a positive integer. The receiver performs spatial/temporal decodingon signals received by M reception antennas through the channels toobtain reception data without crosstalk between the streams (see, forexample, JP-A-2002-44051, hereinafter referred to as Patent Document 1),where M is a positive integer. Ideally, spatial streams are formedcorresponding to a fewer number of transmission and reception antennas(i.e. MIN[N, M]).

According to MIMO communication, a transmission capacity can beincreased according to the number of antennas, and a communication speedcan be improved without increasing frequency bands. Because spatialmultiplexing is used, spectrum efficiency is high. MIMO communicationuses channel characteristics and is different from a simpletransmission/reception adaptive array. For example, in IEEE 802.11n,which is a standard extended from IEEE 802.11a/g, an OFDM_MIMO methodusing OFDM as the primary modulation is employed. Currently, IEEE802.11n is standardized in Task Group n (TGn), in which a specificationis established based on a specification established in Enhanced WirelessConsortium (EWC) formed in October, 2005.

In MIMO communication, in order to spatially divide a spatiallymultiplexed reception signal y into stream signals x, a channel matrix Hmay be acquired by any method and spatially multiplexed reception signaly may be spatially divided into a plurality of original streams usingchannel matrix H by a predetermined algorithm.

Channel matrix H is obtained by allowing a transmitter/receiver totransmit/receive existing training series, estimating channels by adifference between the actually received signal and the existing series,and arranging propagation channels in a matrix form according to acombination of transmission and reception antennas. When there are Ntransmission antennas and M reception antennas, the channel matrix is anM×N (row×column) matrix. Accordingly, the transmitter transmits Ntraining series and the receiver acquires channel matrix H using thereceived training series.

A method for spatially dividing a reception signal is generallyclassified into an open loop type method, in which a receiverindependently performs spatial division on the basis of channel matrixH, and a closed loop type method, in which a transmitter gives weight totransmission antenna on the basis of channel matrix H to performadequate beamforming toward a receiver to form an ideal spatialorthogonal channel.

For an open loop type MIMO transmission method, there is a zero force(see, for example, A. Benjebbour, H. Murata, and S. Yoshida,“Performance comparison of ordered successive receivers for space-timetransmission,” Proc. IEEE VTC Fall, vol. 4, pp. 2053-2057, AtlanticCity, USA, September 2001, hereinafter referred to as Non-PatentDocument 2) or a minimum mean square error (MMSE) (see, for example,“http://radio3.ee.uec.ac.jp/MIMO(IEICE_TS).pdf” (Oct. 24, 2003),hereinafter referred to as Non-Patent Document 3). The open loop typeMIMO transmission method is a relatively simple algorithm for obtainingreception weight matrix W for spatially dividing the reception signalfrom channel matrix H, in which a feedback operation for sharing thechannel information between the transmitter and the receiver is omitted,and the transmitter and the receiver independently perform spatialmultiplexing transmission.

For an ideal closed loop type MIMO transmission method, a singular valuedecomposition (SVD)-MIMO method using SVD of channel matrix H is known(see, for example A. Benjebbour, H. Murata, and S. Yoshida, “Performanceof iterative successive detection algorithm for space-timetransmission,” Proc. IEEE VTC Spring, vol. 2, pp. 1287-1291, Rhodes,Greece, May 2001, hereinafter referred to as Non-Patent Document 1). Inthe SVD-MIMO transmission, a numerical matrix having channel informationthat uses antenna pairs as elements, that is, a channel informationmatrix H, is subjected to the singular value decomposition to obtainUDV^(H). A transmitter uses V in a transmission antenna weight matrix,and transmits a beamformed packet to a receiver. A receiver typicallyuses (UD)⁻¹ as a reception antenna weight matrix. Here, D is a diagonalmatrix having square roots of singular values λ_(i) corresponding toqualities of the spatial streams in diagonal elements (the subscript “i”indicates the i-th spatial stream). Singular values λ_(i) are thediagonal elements of diagonal matrix D in ascending order. Power ratiodistribution or modulation method allocation is performed according tocommunication quality represented by the level of singular value withrespect to the streams, such that a plurality of spatial orthogonalmultiplexed propagation channels, which are logically independent, arerealized. The receiver can extract a plurality of original signal serieswithout crosstalk, and theoretically accomplish maximum performance.

In the closed loop type MIMO communication system, adequate beamformingis performed when the transmitter transmits a packet, but information onthe channel information needs to be fed back from the receiver forreceiving the packet.

For example, in EWC HT (High Throughput) MAC (Media Access Control)Specification Version V1.24, two kinds of procedures, namely, “implicitfeedback” and “explicit feedback,” are defined as the procedure forfeeding back the information on the channel matrix between thetransmitter and the receiver.

For “implicit feedback,” the transmitter estimates a backward channelmatrix from the receiver to the transmitter using training seriestransmitted from the receiver, and a forward channel matrix from thetransmitter to the receiver is computed to perform beamforming under theassumption that bi-directional channel characteristics between thetransmitter and the receiver are reciprocal. Calibration of an RFcircuit in a communication system is performed such that the channelcharacteristics are reciprocal.

For “explicit feedback,” the receiver estimates a forward channel matrixfrom the transmitter to the receiver using training series transmittedfrom the transmitter, and returns a packet including the channel matrixas data to the transmitter. The transmitter performs beamforming usingthe received channel matrix. Alternatively, the receiver computes atransmission weight matrix for allowing the transmitter to performbeamforming from an estimation channel matrix, and returns a packetincluding the transmission weight matrix as the data to the transmitter.For explicit feedback, the channels may not be assumed to be reciprocal,because the weight matrix is computed on the basis of the estimatedforward channel matrix.

In view of packet transmission, the transmitter is an initiator and thereceiver is a terminator. However, in view of beamforming, the initiatorfor transmitting the packet is a beamformer and the terminator forreceiving the beamformed packet is a beamformee. Communication from thebeamformer to the beamformee is referred to as “forward,” andcommunication from the beamformee to the beamformer is referred to as“backward.”

For example, when an access point (AP) transmits a data frame to aclient terminal (STA) as the beamformer, explicit feedback requires thatthe client terminal as the beamformee may only return the trainingseries to the access point for beamforming.

A frame exchange procedure for transmitting the beamforming from theaccess point to the client terminal by implicit feedback will bedescribed with reference to FIG. 8.

First, the access point requests the client terminal to transmittraining series. According to the EWC MAC specification, a linkadaptation control field (illustrated in FIG. 10) of an HT control field(illustrated in FIG. 9) of an MAC frame includes a training request bitTRQ. A value of 1 in training request bit TRQ corresponds to atransmission request of the training series.

The client terminal returns a sounding packet. The sounding packetincludes the training series corresponding to N transmission antennas ofthe access point and M reception antennas of the client terminal. Theaccess point can estimate an N×M backward channel matrix when receivingthe sounding packet. The access point computes a forward transmissionweight matrix for beamforming using the SVD, an Eigen valuedecomposition (EVD) method, or other matrix decomposition methods, andmultiplies transmission signal from the antennas by the transmissionweight matrix, such that the beamformed packet can be sent to the clientterminal. By beamforming, the client terminal may perform wirelesscommunication at a high transmission rate, even if the client terminalis located at a place where it is difficult to receive the packet in thepast.

Subsequently, an operation for allowing the beamformer to performbeamforming using the training series from the beamformee according toimplicit feedback will be described with reference to FIG. 11. In FIG.11, an STA-A having three antennas is a beamformer and an STA-B havingtwo antennas is a beamformee. Hereinafter, a subscript AB indicatesforward transmission from STA-A to STA-B and a subscript BA indicatesbackward transmission from STA-B to STA-A. A numerical subscriptcorresponds to an antenna number of the corresponding terminal. It isassumed that the channels between STA-A and STA-B are reciprocal.Accordingly, a backward channel matrix H_(BA) becomes a transposedforward channel matrix H_(AB) (i.e. H_(BA)=H_(AB) ^(t)).

The training series transmitted from the antennas of STA-B are (t_(BA1),t_(BA2)), and the signals received by the antennas of STA-A through achannel H_(BA) are (r_(BA1), r_(BA2), r_(BA3)). The following equation(1) is obtained.

$\begin{matrix}{\begin{pmatrix}r_{{BA}\; 1} \\r_{{BA}\; 2} \\r_{{BA}\; 3}\end{pmatrix} = {H_{BA}\begin{pmatrix}t_{{BA}\; 1} \\t_{{BA}\; 2}\end{pmatrix}}} & (1)\end{matrix}$

where, channel matrix H_(BA) is a 3×2 matrix expressed by equation (2).Here, h_(ij) is a channel characteristic value of the j-th antenna ofSTA-B with respect to the i-th antenna of STA-A.

$\begin{matrix}{H_{BA} = \begin{pmatrix}h_{11} & h_{12} \\h_{21} & h_{22} \\h_{31} & h_{32}\end{pmatrix}} & (2)\end{matrix}$

When channel matrix H_(BA) is subjected to singular value decomposition,equation (3) is obtained. Here, U_(BA) is a matrix having an inherentnormalized vector of H_(BA)H_(BA) ^(H), V_(BA) is an inherent normalizedvector of H_(BA) ^(H)H_(BA) and D_(BA) is a diagonal matrix having asquare root of an inherent vector of H_(BA)H_(BA) ^(H) or H_(BA)^(H)H_(BA) as diagonal elements. In addition, U_(BA) and V_(BA) areunitary matrices, namely complex conjugate of a transposed matrixbecomes the inverse of the matrix.H _(BA) =U _(BA) D _(BA) V _(BA) ^(H)  (3)

The transmission weight matrix necessary for performing beamforming ofthe frame transmitted from STA-A to STA-B is matrix V_(AB) obtained byperforming the singular value decomposition with respect to forwardchannel matrix H_(AB). Here, because the channels between STA-A andSTA-B are reciprocal and backward channel matrix H_(BA) becomes thetransposed matrix of forward channel matrix H_(AB), the singular valuedecomposition of channel matrix H_(AB) is computed in equation (4).

$\begin{matrix}\begin{matrix}{H_{AB} = {U_{AB}D_{AB}V_{AB}^{H}}} \\{= {V_{BA}^{*}D_{BA}U_{BA}^{T}}}\end{matrix} & (4)\end{matrix}$

When the reciprocity of channels is used, a desired transmission weightmatrix V_(AB) is expressed by equation (5).

$\begin{matrix}\begin{matrix}{V_{AB} = ( V_{AB}^{H} )^{H}} \\{= ( U_{BA}^{T} )^{H}} \\{= ( ( U_{BA}^{T} )^{T} )^{*}} \\{= U_{BA}^{*}}\end{matrix} & (5)\end{matrix}$

That is, it is possible to perform beamforming using the complexconjugate of matrix U_(BA) obtained by performing the singular valuedecomposition with respect to the channel matrix estimated on the basisof the training signal from STA-B.

If the transmission signal of STA-A is x and a reception signal fromSTA-B is y, reception signal y becomes H_(AB)x (i.e. y=H_(AB)x) in acase where the beamforming is not performed (un-steered). If thebeamforming are performed by the transmission weight matrix V_(AB)(steered), reception signal y is obtained in equation (6).

$\begin{matrix}\begin{matrix}{y = {H_{AB}V_{AB}x}} \\{= {{( {U_{AB}D_{AB}V_{AB}^{H}} ) \cdot V_{AB}}x}} \\{= {U_{AB}D_{AB}x}}\end{matrix} & (6)\end{matrix}$

Accordingly, STA-B can perform spatial division of the original streamby multiplying the reception signals by D_(AB) ⁻¹U_(AB) ^(H) as areception weight.

Subsequently, the explicit feedback will be described. In explicitfeedback, the beamformer can receive the explicit feedback of theestimation channel matrix from the beamformee. The format of thefeedback of the estimation channel matrix is generally classified into acase where an MIMO channel coefficient is sent, and a case wheretransmission weight matrix V for beamforming calculated by thebeamformee is sent. The format of the former is called channel stateinformation (CSI). In this case, the beamformer needs to computetransmission weight matrix V for beamforming by constructing channelmatrix H from the received CSI and by performing the singular valuedecomposition. The latter is generally classified into a case wheretransmission weight matrix V for beamforming is sent in an uncompressedformat, and a case where transmission weight matrix V for beamforming issent in a compressed format.

FIG. 12 shows a frame exchange procedure for transmitting beamformingfrom the access point to the client terminal by explicit feedback.

This procedure is initiated by the access point which sends the soundingpacket including a CSI feedback request.

The client terminal estimates the channel matrix based on the soundingpacket and collects the CSI. The CSI data is included in the packet as aCSI feedback (CFB) and returned to the access point.

The access point computes the transmission weight matrix for beamformingfrom the received CFB and multiplies the transmission signal by thetransmission weight matrix to transmit the beamformed packet to theclient terminal. Even if the access point is located in a place wherewireless communication was difficult to achieve in the past, wirelesscommunication can be accomplished at a high transmission rate bybeamforming.

According to implicit feedback described above, reduced burden on thebeamformee due to the feedback allows the access point (AP) to transmita data frame to client terminal STA as beamformer. However, in thiscase, the terminal, which is the beamformer, computes the transmissionweight matrix for beamforming by performing the singular valuedecomposition or other calculation method with respect to the channelmatrix estimated from the received training series. This calculation,however, has a heavy processing load, and the processing load increasesdepending on the increase of the number of streams of the trainingseries transmitted from the beamformee.

In an example shown in FIG. 11, STA-A includes three antennas (N=3), andSTA-B includes two antennas (M=2). Because there are more antennas inSTA-A than in STA-B, no problem is caused in the processing capabilityfor beamforming. This is because STA-A is designed to include theprocessing capability corresponding to N of its own streams; thetraining series of the spatial streams of N or less are divided; an N×Mchannel matrix is constructed from the divided training series; and thematrix for beamforming is computed based on the N×M channel matrix.

However, for N<M, that is, the number of antennas of the beamformee islarger than that of the beamformer, problems may be caused because thebeamformer does not include the processing capability which exceeds thenumber of its own spatial streams. When STA-A can process only Nstreams, which is equal to the number of antennas, M stream trainingsmay not be divided or the matrix for beamforming may not be obtainedfrom the N×M estimation channel matrix.

In order to solve such problems without deteriorating the beamformingcharacteristics, it may be considered that a channel estimation maximumdimension M_(max) corresponding to a rated maximum number of antennas isgiven to STA-A as the beamformer (for example, if it is based on theIEEE specification, M_(max)=4) and the processing capability forcomputing the transmission weigh matrix for beamforming is given to theobtained N×M_(max) estimation channel matrix.

For example, when STA-A includes two antinnas (i.e. N=2) and the ratedmaximum number of antennas is M_(max)=4, STA-A can compute only a 2×2matrix for communication with the terminal having the same number ofantennas, but needs to compute a 2×4 matrix. In this case, calculationor processing circuit needs to be doubled, which renders it difficult toreduce the size and the cost of the communication apparatus.

SUMMARY

It is thus desirable to provide a wireless communication system,wireless communication apparatus, and wireless communication method,which are capable of performing communication at a high transmissionrate using a beamformed packet by allowing a terminal, which is operatedas a beamformer, to divide a spatial stream training transmitted from aterminal, which is operated as a beamformee, to construct an estimationchannel matrix from the divided training series, and to suitably obtaina transmission channel matrix for beamforming on the basis of thechannel matrix.

It is also desirable to provide a wireless communication system,wireless communication apparatus, and wireless communication method,which are capable of performing beamforming without deterioratingbeamforming characteristics, or increasing a processing capability ofchannel estimation or a computing capability of a matrix for beamformingin the beamformer even when a number of antennas of a terminal, namely abeamformer, is smaller than that of a beamformee.

According to an embodiment consistent with the invention, there isprovided a wireless communication system, which performs datatransmission from a first terminal including N antennas to a secondterminal including M antennas using spatially multiplexed streams (N isan integer of 2 or more and M is an integer of 1 or more). The systemincludes notifying means for notifying the second terminal of a channelestimation maximum dimension M_(max) of the first terminal; trainingmeans for transmitting a packet including training series for exciting abackward channel matrix having N rows, and M_(max) or less columns fromthe second terminal to the first terminal in correspondence with thechannel estimation maximum dimension M_(max) of the first terminal andthe number N of antennas of the first terminal; transmission weightmatrix computation means for dividing the training series received bythe antennas of the first terminal into M_(max) or less streams toprepare the backward channel matrix, thereby obtaining a transmissionweight matrix for beamforming at the time of forward data transmissionusing the backward channel matrix; and beamforming means for performingbeamforming in transmission signals from the antennas of the firstterminal using the transmission weight matrix for beamforming, when adata packet is transmitted from the first terminal to the secondterminal.

The term “system” described herein indicates a logical set ofapparatuses, or function modules for realizing specific functions. It isto be understood that the apparatuses or the function modules are notnecessarily included in a single casing (the same is true in the belowdescriptions).

In order to realize high speed wireless communications, there isprovided an MIMO communication method which enables wirelesscommunications using spatially multiplexed streams between a transmitterand a receiver, both the transmitter and the receiver including aplurality of antenna elements. In particular, in a closed loop type MIMOcommunication system, a terminal of a data packet transmission sideperforms beamforming on the basis of feedback of information on anestimation channel matrix from a terminal of a reception side, such thata plurality of spatially orthogonal multiplexed propagation channels,which are logically independent, are realized. The receiver side canextract a plurality of original signal series without crosstalk, therebytheoretically accomplishing maximum performance.

As a procedure of performing feedback of the channel matrix from theterminal of the reception side to the terminal of the transmission side,for example, two kinds of procedures, that is, “implicit feedback” and“explicit feedback,” are defined in the EWC HT MAC specification. Amongthem, in the implicit feedback, the first terminal, which is operated asa beamformer, divides a spatial stream training transmitted from asecond terminal, which is operated, as a beamformee, constructs abackward estimation channel matrix from the divided training series, andperforms beamforming of a transmission packet using a transmissionchannel matrix for beamforming obtained on the basis of the channelmatrix to perform communication, by assuming that the bi-directionalchannel characteristics between the transmitter and the receiver arereciprocal.

For example, when an access point transmits a data frame to a clientterminal as the beamformer, according to the implicit feedback, theclient terminal as the beamformee only returns the training series tothe access point, in order to perform the beamforming.

However, in a case of N<M, that is, the number of antennas of the secondterminal is larger than that of the first terminal, because the firstterminal which is operated as the beamformer does not include theprocessing capability which exceeds the number of its own spatialstreams, the first terminal may not divide M stream trainings or obtainthe matrix for beamforming from the N×M estimation channel matrix.

In the wireless communication system according to an embodimentconsistent with the invention, when the beamforming based on thebackward channel estimation result is performed according to theimplicit feedback, the channel estimation maximum dimension M_(max) ofthe first terminal is previously notified to the second terminal, andthe second terminal transmits the packet including the training seriesfor exciting the N×M_(max) backward channel matrix in correspondencewith the channel estimation maximum dimension M_(max) of the firstterminal and the number N of antennas of the first terminal. In otherwords, the second terminal returns the training series which suppressesthe number of streams to be less than or equal to the channel estimationmaximum dimension M_(max) of the first terminal. Accordingly, the firstterminal divides the spatial stream training of the training seriesreceived in a range of processing capability corresponding to the numberof its own antennas, and constructs the backward channel matrix from thedivided training series, thereby obtaining the transmission weightmatrix for beamforming.

For example, when the wireless communication system is based on the EWCHT MAC specification, the first terminal requests the training seriesthrough a TRQ bit included in the link adaptation control field of theHT control field of the MAC frame. The second terminal suppresses thenumber of the streams to be less than or equal to the channel estimationmaximum dimension M_(max) of the first terminal, and transmits thesounding packet.

Accordingly, in an embodiment consistent with the invention, when aclosed loop type MIMO communication is performed by the implicitfeedback, the first terminal, which is operated as the beamformer, canperform channel estimation having a dimension number. The dimensionnumber is suppressed according to the number of its own antenna, and thetransmission weight matrix for beamforming is computed with thesuppressed dimension number, thereby reducing the circuit size of thefirst terminal. In more detail, the size of the circuit module forestimating the channel matrix can be reduced to an order of about (N/M)²and the size of the circuit of the beamforming transmission weightmatrix computation means can be reduced to an order of about (N/M)².

In an embodiment consistent with the invention, the method for notifyingthe channel estimation maximum dimension M_(max) of the first terminalto the second terminal is not limited. For example, it may be consideredthat the spatial dimension is specified in the packet for requesting thesounding packet. However, in the defined HT control field, a surplus bitfield does not exist. Accordingly, when a bit is newly added to thefield, overhead may increase.

Meanwhile, in the EWC specification, it is defined that any HT functionsupported by a HT terminal is transmitted as the HT capability elementand is declared. In the HT capability element, a transmit beamforming(TxBF) capability field for describing the existence of the support ofany HT function for beamforming is provided. Accordingly, when theterminal, which is operated as the beamformee, performs the explicitfeedback, a capability description field for describing the spatialdimension number of the sounding packet, which can be received from thebeamformer, is included.

Accordingly, in an embodiment consistent with the invention, the channelestimation maximum dimension, which can be received from the beamformeein the implicit feedback, is described in the capability descriptionfield, regardless of whether the wireless communication apparatuscorresponds to the explicit feedback or not.

When the wireless communication apparatus does not correspond to theexplicit feedback, the capability description field is generally unused(N/A). When the wireless communication apparatus corresponds to theexplicit feedback, the maximum spatial dimension, when the beamformeereceives the sounding packet, is described, which is equivalent to themaximum spatial dimension receivable by the beamformee in the implicitfeedback.

Accordingly, although, in the explicit feedback, the spatial dimensionnumber of the sounding packet receivable from the beamformer is used asthe maximum spatial dimension when receiving the sounding packet,regardless of whether the explicit feedback is supported or not, noproblem is caused.

The capability description field is originally used for detecting thechannel estimation maximum dimension of the beamformee to which thebeamformer transmits the sounding packet in the explicit feedback.Although a method of analyzing the capability description field whenperforming the implicit feedback is not defined in the standardspecification, an equivalent transmission operation can be performedbetween specific types as proprietary signaling. It is possible toadequately suppress the number of streams of the sounding packet byperforming the method of analyzing the capability description field inthe reception side of the beamforming when performing the implicitfeedback. Although an example of a method for notifying a maximumspatial dimension when receiving the sounding packet using a field,which is already defined in the EWC specification from the firstterminal to the second terminal, is described herein, the invention isnot limited thereto. For example, the same effect can be obtained byallocating two bits of reserved bits, which exist in the EWCspecification, to a bit field indicating the maximum spatial dimensionwhen receiving the sounding packet.

The HT capability element may be included in a predetermined managementframe. For example, when the wireless communication apparatus isoperated as an access point, the HT capability element may be includedin a type of transmission frame such as a beacon signal which isnotified in a frame period, a measure pilot for measuring acommunication link, an association response and a re-associationresponse which respond to the request of association from the clientterminal, or a probe response which responds to the request of basicservice set (BSS) information from the client terminal. In addition,when the wireless communication apparatus is operated as the clientterminal (or a communication station other than an access point), the HTcapability element may be included in a type of transmission frame of anassociation request and re-association request for requesting networkassociation to the access point, and a probe request for requesting BSSinformation to the access point.

The notifying means can notify the channel estimation maximum dimensionof the beamformer by the implicit feedback, using the existing bitfield, without increasing the overhead on the protocol.

According to an embodiment consistent with the invention, there isprovided a wireless communication system, a wireless communicationapparatus, and a wireless communication method, which are capable ofperforming communication at a high transmission rate by a beamformedpacket. The high transmission rate is achieved by allowing a terminal,which is operated as a beamformer, to divide spatial stream trainingseries transmitted from a terminal, which is operated as a beamformee,to construct an estimation channel matrix from the divided trainingseries, and to suitably obtain a transmission channel matrix forbeamforming on the basis of the channel matrix.

According to an embodiment consistent with the invention, there isprovided a wireless communication system, a wireless communicationapparatus, and a wireless communication method, which are capable ofsuitably performing beamforming without increasing a processingcapability of channel estimation, or a computing capability of a matrixfor beamforming in the beamformer, even when the number of antennas of aterminal, which is a beamformer, is smaller than that of a beamformee.

In the wireless communication system, according to an embodimentconsistent with the invention, when the beamforming is performed on thebasis of a backward channel estimation result by the implicit feedback,and the number of antennas of a terminal of a transmitter side issmaller than that of a terminal of a receiver side, estimation of achannel, in which the dimension is suppressed, and computation of atransmission weight matrix for beamforming, in which the dimension issuppressed, are possible by previously notifying the terminal at thetransmitter side with a channel estimation maximum dimension, and thusthe circuit size of the terminal of the transmitter side can be reduced.

Other objects, features, and advantages consistent with the inventionwill become apparent and more readily appreciated from the followingdescriptions, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an operation procedure of implicitfeedback, according to an embodiment.

FIG. 2 schematically illustrates a transmitter of a wirelesscommunication apparatus which can be an STA-A (or STA-B) of FIG. 1.

FIG. 3 schematically illustrates a receiver of the wirelesscommunication apparatus which can be the STA-A (or STA-B) of FIG. 1.

FIG. 4 shows a format of a HT capability element defined in IEEE 802.11nstandard.

FIG. 5 shows a format of a Tx beamforming capability field included inthe HT capability element.

FIG. 6 is a flowchart illustrating a process when the wirelesscommunication apparatuses shown in FIGS. 2 and 3 operate as a beamformeron the basis of the implicit feedback.

FIG. 7 is a flowchart illustrating a process when the wirelesscommunication apparatuses shown in FIGS. 2 and 3 operate as a beamformeeon the basis of the implicit feedback.

FIG. 8 schematically illustrates a frame exchange procedure fortransmitting beamforming from an access point to a client terminal bythe implicit feedback.

FIG. 9 schematically illustrates a format of a HT control field of anMAC frame defined in IEEE 802.11 standard.

FIG. 10 schematically illustrates a format of a link adaptation controlfield in the HT control field shown in FIG. 9.

FIG. 11 schematically illustrates a calculation process for allowing thebeamformer to perform the beamforming using training series from thebeamformee according to the implicit feedback.

FIG. 12 schematically illustrates a frame exchange procedure fortransmitting beamforming from an access point to a client terminal byexplicit feedback.

FIG. 13 schematically illustrates an aspect of using two bits of B27 toB28 of a Tx beamforming capability field as a “maximum channelestimation dimension at receiving” field.

DETAILED DESCRIPTION

Hereinafter, embodiments consistent with the invention will be describedin detail with reference to the accompanying drawings.

A wireless communication system of one embodiment performs closed looptype MIMO communication. In particular, a terminal at a transmitter sideperforms beamforming by performing feedback for a channel matrix, suchas the “implicit feedback” defined in the EWC HT MAC specification.

For implicit feedback, a terminal operated as a beamformer divides aspatial stream training transmitted from a terminal operated as abeamformee, constructs a backward estimation channel matrix from thedivided training series, and performs beamforming of a transmissionpacket using a transmission channel matrix for beamforming obtained onthe basis of the channel matrix, thereby performing communication.

However, a processing capability for performing channel estimation or aprocessing capability for computing a matrix for beamforming of aterminal is determined according to the number of antennas of theterminal. Accordingly, if the number of antennas of the beamformee islarge, transmitted spatial stream training exceeds a channel estimationmaximum dimension allowed by the terminal. As a result, the spatialstream training may not be divided or a matrix for beamforming may notbe computed from a channel matrix of a higher dimension.

In contrast, in the wireless communication system according to theembodiment, the beamformee is pre-notified of the channel estimationmaximum dimension of the beamformer. Accordingly, when the beamformeereceives a transmission request of training series from the beamformer,the number of streams at the time of transmitting the training series issuppressed to an antenna estimation maximum dimension. Thus, thebeamformer divides the spatial stream training of the training seriesreceived in a range of its own processing capability, constructs abackward channel matrix from the divided training series, and extracts atransmission weight matrix for beamforming.

FIG. 1 schematically illustrates an operation procedure of the implicitfeedback according to the embodiment. Here, the number of antennas of anSTA-A as the beamformer or a channel estimation maximum dimension istwo, and the number of antennas of an STA-B as the beamformee is three.The procedure is performed on the basis of the EWC MAC specification.

First, STA-A requests STA-B to transmit training series. A requestpacket uses a 3×2 channel. In a transmission capability of STA-A and areception capability of STA-B, the number of spatial streams used by thetransmission capability of STA-A is restricted to one or two spatialstreams.

STA-B returns a sounding packet including the training series inresponse to the request packet. When this packet is transmitted, thenumber of spatial streams is suppressed to the channel estimationmaximum dimension of STA-A and a 2×2 backward channel is excited.Accordingly, when STA-A receives the sounding packet, STA-A can generatea 2×2 backward estimation channel matrix. In such a case, STA-A canperform the channel estimation and the computation of the transmissionweight matrix for beamforming in the range of the assumed processingcapability and the size of the circuit of the terminal can be reduced.

Thereafter, the request of sounding packet, the channel estimation, andthe computation of transmission weight matrix for beamforming due to thereception of sounding packet are repeatedly performed whenever STA-Aperforms the beamforming.

Hereinafter, a wireless communication system according to an embodimentconsistent with the invention will be described in detail.

FIGS. 2 and 3 schematically illustrate a transmitter and a receiver of awireless communication apparatus. The transmitter and the receiver maybe operated as STA-A and STA-B of the wireless communication systemshown in FIG. 1, respectively. STA-A may include N antennas. In oneexample, N is four or less, according to IEEE specification. However,only two antennas are shown in the FIGS. 2 and 3 in order to avoidconflict of illustration.

Transmission data supplied from a data generator 100 is scrambled by ascrambler 102. Subsequently, error correction encoding is performed byan encoder 104. For example, in the EWC HT PHY specification, scramblingand encoding methods are defined according to the definition of IEEE802.11a. The encoded signal is input to a data division unit 106 to bedivided into transmission streams.

In each transmission stream, a transmission signal is punctured by apuncture 108 according to a data rate applied to each stream,interleaved by an interleaver 110, mapped to an IQ signal space by amapper 112, thereby becoming a conjugate baseband signal. A selector 111inserts the training series into the transmission signal of eachinterleaved spatial stream at an adequate timing and supplies it tomapper 112. In the EWC HT PHY specification, an interleaving schemeexpands the definition of IEEE 802.11a, such that the same interleavingis not performed among a plurality of streams. For mapping scheme, BPSK,QPSK, 16QAM, or 64QAM is applied according to IEEE 802.11a.

When beamforming is performed with respect to the transmission signal,in a spatial multiplexer 114, a beamforming transmission weight matrixcomputation unit 114 a calculates transmission weight matrix V forbeamforming from channel matrix H using a computation method such as thesingular value decomposition. A transmission weight matrixmultiplication unit 114 b multiplies the transmission vector having thetransmission streams as its elements by transmission weight matrix V,thereby performing the beamforming. When transmitting the soundingpacket, the beamforming is not performed with respect to thetransmission signal. The beamforming transmission weight matrixcomputation unit 114 a computes the transmission weight matrix byequations (3) to (5) using the backward channel matrix constructed byallowing a channel matrix estimation unit 216 a (described below withreference to FIG. 3) of the receiver to divide the spatial streamtraining transmitted from the beamformee.

An inverse fast Fourier transform unit (IFFT) 116 converts thesubcarriers arranged in a frequency region into a time axis signal. Aguard insertion unit 118 adds a guard interval. A digital filter 120performs band limitation, a Digital-Analog converter (DAC) 122 convertsthe band-limited signal into an analog signal, and an RF unit 124up-converts the analog signal to an adequate frequency band andtransmits the converted signal to the channel through a transmissionantenna. In implicit feedback, assuming that the channel characteristicsare reciprocal, RF unit 124 performs calibration.

Meanwhile, the data that reaches the receiver (shown in FIG. 3) throughthe channel is analog-processed in an RF unit 228, converted into adigital signal by an Analog-Digital converter (ADC) 226, and input to adigital filter 224, in each reception antenna branch. In implicitfeedback, assuming that the channel characteristics are reciprocal, RFunit 228 performs calibration.

Subsequently, a synchronization circuit 222 performs processes includingpacket detection, timing detection, and frequency offset correction. Aguard removing unit 220 removes the guard interval added to the top ofdata transmission section. A fast Fourier transform unit (FFT) 218transforms a time domain signal into a frequency domain signal.

A space division unit 216 performs a space division process of thespatially multiplexed reception signal. In particular, a channel matrixestimation unit 216 a divides the spatial stream training included inthe PHY header of the sounding packet and constructs an estimationchannel matrix H from the training series. In implicit feedback, whenthe apparatus operates as a beamformer, estimation channel matrix Hobtained by channel matrix estimation unit 216 a is sent to beamformingtransmission weight matrix computation unit 114 a of the transmitter asa backward channel matrix. In the implicit feedback procedure, when theapparatus operates as a beamformee, an antenna reception weight matrixcomputation unit 216 b computes an antenna reception weight matrix W onthe basis of channel matrix H obtained by channel matrix estimation unit216 a. In a case that beamforming is performed with respect to thereception packet and that the estimation channel matrix is subjected tothe singular value decomposition, the estimation channel matrix becomesUD (see Equation (6)), and antenna reception weight W is calculatedtherefrom. Although antenna reception weight W is calculated using thesingular value decomposition, it is appreciated that other calculationmethods, such as zero forcing or MMSE, may be used. An antenna receptionweight matrix multiplication unit 216 c multiplies the reception vectorhaving the reception streams as its elements by antenna reception weightmatrix W to perform spatial decoding of the spatial multiplexed signal,thereby obtaining independent signal series for each stream.

A channel equalization circuit 214 performs remaining frequency offsetcorrection and channel tracking with respect to the signal series ofeach stream. A demapper 212 demaps the reception signal on the IQ signalspace, a deinterleaver 210 performs deinterleaving, and a depuncture 208performs depuncturing at a predetermined data rate.

A data synthesis unit 206 synthesizes a plurality of reception streamsto one stream. This data synthesis process performs an operation opposedto the data division performed in the transmitter. A decoder 204performs error correction decoding, a descrambler 202 performsdescrambling, and a data acquiring unit 200 acquires the reception data.

When the wireless communication apparatus operates as a datatransmission terminal, that is, the beamformer, in the closed loop typeMIMO communication, the beamformer pre-notifies the beamformee ofchannel estimation maximum dimension M_(max), that is, the maximumspatial dimension of the sounding packet that can be received by thebeamformee. Generally, channel estimation maximum dimension M_(max) isequal to the number N of antennas of the wireless communicationapparatus as the beamformer (the channel estimation maximum dimension isone of the capability of the wireless communication apparatus, and aprocedure of notifying of the channel estimation maximum dimension willbe described later). When beamforming is performed to start thetransmission of data packet or when the transmission weight matrix forbeamforming is desired to be updated, a training request TRQ is issuedto the beamformee.

When the wireless communication apparatus operates as a beamformee, thesounding packet for exciting the wireless propagation channel istransmitted in response to the reception of training request. Here, whenthe pre-notified channel estimation maximum dimension N of thebeamformer is smaller than the number M of antennas of the beamformee,the beamformee restricts the spatial dimension of sounding packet tochannel estimation maximum dimension M_(max) of the beamformer.

When the beamformer receives the sounding packet, the beamformer dividesthe spatial stream training transmitted from the beamformee andconstructs the backward estimation channel matrix from the dividedtraining series. Even when the number M of antennas of the beamformee islarger than the number N of antennas of the beamformer, the spatialdimension of sounding packet is restricted to channel estimation maximumdimension N(=M_(max)) of the beamformer. The beamformer estimates an N×Nchannel matrix as a maximum, and N×N transmission weight matrix forbeamforming is computed from the N×N maximum estimation channel matrixusing a computation method, such as the singular value decomposition.

Accordingly, when the wireless communication apparatus is configured tobe a beamformer, the circuit size of channel matrix estimation unit 216a can be reduced to the order of about (N/M)². Comparing with the casewhere the N×N transmission weight matrix for beamforming is computedfrom the N×M maximum channel matrix estimation result, the circuit sizeof beamforming transmission weight matrix computation unit 216 b can bereduced to the order of about (N/M)². Because the circuit configurationrelated to spatial division and spatial multiplexing is complicated, thespatial dimension of sounding packet is restricted, so as to accomplishminiaturization, low cost, and low power consumption of the wirelesscommunication apparatus.

In order to perform the above-described beamforming procedure, thebeamformee needs be pre-notified of channel estimation maximum dimensionM_(max). Hereinafter, a method for notifying the channel estimationmaximum dimension according to the EWC specification will be described.

For example, when a beamformer requests that the sounding packetincluding the training series be transmitted, a method for notifyingchannel estimation maximum dimension M_(max) may be used. However, therequest of training series is performed using only the training request(TRQ) included in the link adaptation control field (shown in FIG. 10)in the HT control field (shown in FIG. 9) of the MAC frame. Anadditional field which can be used in the description of the channelestimation maximum dimension does not exist. New definition of the frameformat including the description field of channel estimation maximumdimension is not realized in a standardized operation, and is notefficient because adding a new bit field increases communicationoverhead.

In implicit feedback, the beamformee only returns the sounding packetincluding the training series according to the beamformer's request.However, in explicit feedback, the beamformee performs the channelestimation. Although the beamformer for requesting the explicit feedbacktransmits the sounding packet with the number of spatial streamsexceeding the channel estimation maximum dimension of the beamformer,the beamformee does not divide the sounding packet, and does notconstruct the channel matrix nor compute the transmission weight matrix.Thus, the feedback is inadequate.

Accordingly, in the EWC specification, when explicit feedback isapplied, a method for notifying information of the channel estimationmaximum dimension is determined as one of the capability of beamformee.

Accordingly, the beamformee can be notified of the spatial dimension ofsounding packet received when operating the beamformer in implicitfeedback, using the method for notifying the capability of beamformee,regardless of whether the wireless communication apparatus correspondsto explicit feedback or not. Hereinafter, the notifying method will bedescribed.

In the EWC specification, interconnection with IEEE 802.11a/b/g ismaintained, but it is defined that a terminal, which transmits the HTcapability element in high-speed, is declared to be the HT terminal. TheHT terminal may include the HT capability element in a predeterminedmanagement frame, and declare any element of the HT functions by the HTcapability element.

For example, any HT function determines whether the wirelesscommunication apparatus supports only implicit feedback, or bothimplicit feedback and explicit feedback, with respect to beamforming. Ingeneral, there are three kinds of explicit feedback, which includes theCSI, uncompressed, and compressed (described above), but an HT functionwill determine which format is supported.

FIG. 4 shows a format of HT capability element. As shown, in a transmitbeamforming (TxBF) capability field, an HT function for beamforming isspecified. FIG. 5 shows the configuration of the Tx beamformingcapability field.

The Tx beamforming capability field has 32 bits. Among them, the 19^(th)to the 20^(th) bits are allocated to the CSI number of beamformerantennas, the 21^(th) to the 22^(nd) bits are allocated to theuncompressed steering matrix of beamformer antennas, and the 23^(rd) tothe 24^(th) bits are allocated to the compressed steering matrix ofbeamformer antennas. In these fields, the spatial dimension number ofsounding packet receivable from the beamformer when the beamformeeperforms explicit feedback with each format is described.

In the present embodiment, channel estimation maximum dimension M_(max)receivable by the beamformer from the beamformee in implicit feedback isdescribed in at least one field, regardless of whether the wirelesscommunication apparatus corresponds to explicit feedback or not.

When the wireless communication apparatus does not correspond toexplicit feedback, the 19^(th) to the 24^(th) bit fields of TxBFcapability field is generally unused (N/A). When the wirelesscommunication apparatus corresponds to explicit feedback, a maximumspatial dimension for the beamformee to receive the sounding packet isdescribed, which is equivalent to a maximum spatial dimension receivableby the beamformer in implicit feedback.

Accordingly, regardless of whether explicit feedback is supported ornot, there is no problem to use the spatial dimension number of soundingpacket, which is described in the 19^(th) to the 24^(th) bit fields ofthe TxBF capability field in the explicit feedback and received from thebeamformer, as the maximum spatial dimension when the sounding packet isreceived.

The 19^(th) to the 24^(th) bit fields of the TxBF capability field areoriginally used for detecting the channel estimation maximum dimensionof beamformer to which the beamformer transmits the sounding packet inexplicit feedback. Although a method for analyzing the bit field is notdefined in a standardized specification when performing implicitfeedback, an equivalent transmission operation can be performed betweenspecific types as proprietary signaling. It is possible to adequatelysuppress the number of streams of the sounding packet by performing themethod for analyzing the bit field in the reception side of thebeamforming when performing implicit feedback. Although an example of amethod for notifying a maximum spatial dimension when receiving thesounding packet using a field, which is already defined in the EWCspecification from a first terminal to a second terminal, is describedherein, the invention is not limited thereto. For example, the sameeffect can be obtained by allocating two reserved bits, which exist inthe EWC specification, to a bit field indicating the maximum spatialdimension when receiving the sounding packet. As an additional definingmethod, for example, information of the maximum spatial dimension whenreceiving the sounding packet is described using a partial bit field ofB25 to B31, which is a “reserved” area in the current Tx beamformingcapability field. In particular, two bits of B27 to B28 is used as“maximum channel estimation dimension at receiving” field (see FIG. 13).A matrix having one row and N columns is defined as a maximum, if thevalue thereof is zero; a matrix having two rows and N columns is definedas a maximum, if the value thereof is one; a matrix having three rowsand N columns is defined as a maximum, if the value thereof is two; anda matrix having four rows and N columns is defined as a maximum, if thevalue thereof is three, thereby representing the spatial dimensionallowed when receiving the sounding packet. Here, the channel matrix isbased on a direction from the first terminal to the second terminal. Amatrix at a time point when the channel is estimated in the firstterminal is represented by a matrix having N rows and one column, amatrix having N rows and two columns, a matrix having N rows and threecolumns, and a matrix having N rows and four columns as a maximum.

The HT capability element may be included in a predetermined managementframe. For example, when STA-A operates as the access point, the HTcapability element may be included in a type of transmission frame of abeacon signal, which is notified in a frame period, a measure pilot formeasuring a communication link, both an association response and are-association response, which respond to the request of associationfrom the client terminal, or a probe response, which responds to therequest of Basic Service Set (BSS) information from the client terminalsuch that the dimension of CSI information is notified to STA-B, whichparticipates in the network operated by STA-A. In addition, when STA-Aoperates as a client terminal (or a communication station other than theaccess point), the HT capability element may be included in a type oftransmission frame of an association request and re-association requestfor requesting network association to STA-B, which operates as theaccess point and a probe request for requesting BSS information to theaccess point. Accordingly, even when the wireless communicationapparatus operates as any one of the access point and the clientterminal, the wireless communication apparatus can be notified of thechannel estimation maximum dimension of beamformer by implicit feedbackby transmitting the HT capability element.

By using the existing bit field, it is possible to notify the channelestimation maximum dimension of beamformer by implicit feedback, withoutincreasing the overhead of the protocol.

FIG. 6 is a flowchart illustrating a process when wireless communicationapparatuses shown in FIGS. 2 and 3 operate as an initiator, that is, abeamformer, on the basis of implicit feedback.

First, the apparatus notifies a receiver of channel estimation maximumdimension M_(max) (step S1). In one example, when the wirelesscommunication apparatus operates as an access point, the HT capabilityelement is included in the beacon signal. In another example, when thewireless communication apparatus operates as a client terminal, the HTcapability element is included in the message for the networkassociation for the access point. This notification does not requireimmediacy, and thus does not need to be repeated whenever thetransmission using the beamforming is performed.

Next, the apparatus transmits a request for the training signal to thereceiver which operates as a beamformee (step S2). In more detail, a TRQbit included in the link adaptation control field of the HT controlfield of the MAC frame is placed.

Then, the apparatus receives the sounding packet transmitted from thereceiver in response to the request (step S3). The sounding packetincludes the training series for exciting an N×M_(max) backward channelmatrix in correspondence with channel estimation maximum dimensionM_(max) and N antennas. In other words, the number of streams of thesounding packet is suppressed to the channel estimation maximumdimension.

Next, the training series received by the antennas are divided intoM_(max) streams to prepare the backward channel matrix (step S4) and thetransmission weight matrix for beamforming upon obtaining the forwarddata transmission using the backward channel matrix (step S5).

The beamforming is performed in a transmission vector having thetransmission signals from the antennas as its element using thetransmission weight matrix for beamforming, and the data packet istransmitted to the receiver (step S6). It is possible to make an idealspatially orthogonal channel by weighting the transmission antennas onthe basis of the channel matrix and performing the adequate beamformingdirected to the receiver.

FIG. 7 is a flowchart illustrating a process when the wirelesscommunication apparatuses shown in FIGS. 2 and 3 operate as a receiver,that is, a beamformer, on the basis of implicit feedback.

First, the apparatus receives channel estimation maximum dimensionM_(max) of the initiator (step S11). When the initiator operates as anaccess point, the HT capability element is included in the beaconsignal. When the initiator newly participates in a network as a clientterminal in which the apparatus operates as the access point, the HTcapability element is included in the message for network association.

Next, the apparatus receives the request for the training signal fromthe initiator which operates as a beamformer (step S12). In more detail,the TRQ bit included in the link adaptation control field of the HTcontrol field of the MAC frame received from the initiator is placed.

Then, the apparatus returns the sounding packet to the initiator inresponse to the request (step S13). The sounding packet includes thetraining series for exciting the N×M_(max) backward channel matrix incorrespondence with channel estimation maximum dimension M_(max) and Nantennas. In other words, the number of streams of the sounding packetis suppressed to the channel estimation maximum dimension.

The initiator divides the training series received by the N antennasinto the M_(max) streams to prepare the backward channel matrix, andobtains the transmission weight matrix for beamforming at the time ofthe forward data transmission using the backward channel matrix.Beamforming is performed in the transmission vector having thetransmission signals from the N antennas as its elements using thetransmission weight matrix for beamforming, and the data packet istransmitted to the receiver.

The wireless communication apparatus operated as a beamformee dividesthe spatial stream training received from the initiator, constructs theforward estimation channel matrix (step S14), and obtains the receptionweight matrix from the channel matrix (step S15). A method of computingthe reception weight matrix, a ZF method, or an MMSE method may be used.Alternatively, D⁻¹U^(H) computed from matrices U and D obtained byperforming the singular value decomposition with respect to the channelestimation matrix may also be used.

When the N antennas receive the data packet from the initiator, thereception vector composed of reception signals for the payload part ismultiplied with the reception weight matrix to perform spatial decodingof the spatial multiplexing signal. The signal series, which areindependent in each stream, are obtained (step S16). By beamforming,communication can be performed at a high transmission rate even if thecommunication apparatus is located in a place where the packet wasdifficult to receive in the past.

Although the invention has been described in detail with reference tospecific embodiments, it is apparent to those skilled in the art thatthese embodiments may be modified or substituted without departing fromthe scope consistent with the invention.

Although an embodiment consistent with the invention has been describedas being related to the MIMO communication system according to the EWCspecification set forth in IEEE 802.11n, the scope of the invention isnot limited thereto. Because the MIMO communication system usesspatially multiplexed streams transmitted from a first terminalincluding N antennas to a second terminal including M antennas, it ispossible to apply the invention to various types of communicationsystems, in which a beamformer performs beamforming using a trainingsignal transmitted from a beamformer.

Although, for simplicity, a transmission terminal is described in oneembodiment to perform “direct mapping” for directly mapping the streamsto the antenna branches, the invention is applicable to employ “spatialexpansion” or a conversion method, in which the streams do not have aone-to-one correspondence with the antenna branches.

Although an embodiment applicable to IEEE 802.11n standard, which isextended from IEEE 802.11, is described in the present specification,the invention is not limited thereto. The invention is applicable to avariety of wireless communication systems using an MIMO communicationmethod, such as a mobile WiMax (Worldwide Interoperability forMicrowave) based on IEEE 802.16e standard, a high-speed wirelesscommunication for mobile objects based on IEEE 802.20 standard, ahigh-speed wireless PAN (Personal Area Network) using 60 GHz (milliwave)band based on IEEE 802.15.3c standard, a wireless HD (High Definition)which transmits an uncompressed HD image using wireless transmission of60 GHz (milliwave) band, and a fourth generation (4G) mobile telephone.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations, and alterations may occurdepending on design requirements and other factors. All of suchmodifications, combinations, sub-combinations, and alterations areconsidered within the scope of the appended claims or the equivalentsthereof.

What is claimed is:
 1. A wireless communication system configured toperform data transmission between a first terminal having a firstquantity of antennas and a second terminal having a second quantity ofantennas, the system comprising: notifying means configured to notifythe second terminal of a channel estimation capability of the firstterminal, wherein the channel estimation capability is included in atransmit beamforming capability field and indicates a maximum number ofspace-time stream; and transmitting means configured to transmit signalsfrom the first terminal to the second terminal based on the channelestimation capability, so as to establish a wireless communicationchannel between the first terminal and the second terminal; wherein themaximum number of space-time streams indicated by the channel estimationcapability is used to limit a number of columns of a channel matrix formulti-input multi-output (MIMO) communication, and wherein the signalsare transmitted with band limiting by digital filter processing, anumber of digital filter processing being equal to the first quantity ofantennas.
 2. The wireless communication system of claim 1, furthercomprising: training means configured to send a sounding packet from thesecond terminal to the first terminal, wherein the sounding packetcomprises a training series for preparing the channel matrix for MIMOcommunication having the first quantity of rows and a third quantity ofcolumns, wherein the third quantity is equal to or less than the channelestimation capability.
 3. The wireless communication system of claim 2,further comprising: transmission weight matrix computation meansconfigured to divide the training series into the third quantity ofstreams to prepare the channel matrix for MIMO communication.
 4. Thewireless communication system of claim 2, further comprising:beamforming means configured to beamform signals according to atransmission weight matrix, thereby transmitting a data packet derivedfrom the signals via the wireless communication channel, wherein thetransmission weight matrix is determined according to the channel matrixfor MIMO communication.
 5. The wireless communication system of claim 1,wherein the channel estimation capability is represented by two bits,the two bits being located after bit fields used for detecting a channelestimation capability of the second terminal.
 6. A wirelesscommunication apparatus including a first quantity of antennas fortransmitting data to a receiver including a second quantity of antennas,the apparatus comprising: notifying unit configured to notify thereceiver of a channel estimation capability of the apparatus, whereinthe channel estimation capability is included in a transmit beamformingcapability field and indicates a maximum number of space-time stream;and transmitting unit configured to transmit signals from the apparatusto the receiver based on the channel estimation capability, so as toestablish a wireless communication channel between the apparatus and thereceiver; wherein the maximum number of space-time streams indicated bythe channel estimation capability is used to limit a number of columnsof a channel matrix for multi-input multi-output (MIMO) communication,and wherein the signals are transmitted with band limiting by digitalfilter processing, a number of digital filter processing being equal tothe first quantity of antennas.
 7. The wireless communication apparatusof claim 6, further comprising: training packet receiving unitconfigured to receive a sounding packet from the receiver, wherein thesounding packet comprises a training series for preparing the channelmatrix for MIMO communication having the first quantity of rows and athird quantity of columns, wherein the third quantity is equal to orless than the channel estimation capability.
 8. The wirelesscommunication apparatus of claim 7, further comprising: transmissionweight matrix computation unit configured to divide the training seriesinto the third quantity of streams to prepare the channel matrix forMIMO communication.
 9. The wireless communication apparatus of claim 7,further comprising: beamforming unit configured to beamform signalsaccording to a transmission weight matrix, thereby transmitting a datapacket derived from the signals via the wireless communication channel,wherein the transmission weight matrix is determined according to thechannel matrix for MIMO communication.
 10. The wireless communicationapparatus of claim 6, wherein the channel estimation capability isrepresented by two bits, the two bits being located after bit fieldsused for detecting a channel estimation capability of the receiver. 11.A wireless communication apparatus including a first quantity ofantennas for receiving data from an initiator including a secondquantity of antennas, the apparatus comprising: notification receivingunit configured to receive notification of a channel estimationcapability of the initiator, wherein the channel estimation capabilityis included in a transmit beamforming capability field and indicates amaximum number of space-time stream; and receiving unit configured toreceive signals from the initiator based on the channel estimationcapability, so as to establish a wireless communication channel betweenthe apparatus and the initiator; wherein the maximum number ofspace-time streams indicated by the channel estimation capability isused to limit a number of columns of a channel matrix for multi-inputmulti-output (MIMO) communication, and wherein the signals are receivedwith band limiting by digital filter processing, a number of digitalfilter processing being equal to the first quantity of antennas.
 12. Thewireless communication apparatus of claim 11, further comprising:training packet transmitting unit configured to transmit a soundingpacket to the initiator, wherein the sounding packet comprises atraining series for preparing the channel matrix for MIMO communicationhaving the second quantity of rows and a third quantity of columns,wherein the third quantity is equal to or less than the channelestimation capability.
 13. The wireless communication apparatus of claim12, wherein the channel matrix for MIMO communication is prepared bydividing the training series into the third quantity of streams.
 14. Thewireless communication apparatus of claim 12, wherein the packetreceiving means is further configured to: receive signals beamformed bythe initiator according to a transmission weight matrix, therebyreceiving a data packet derived from the signals via the wirelesscommunication channel, wherein the transmission weight matrix isdetermined according to the channel matrix for MIMO communication. 15.The wireless communication apparatus of claim 11, wherein the channelestimation capability is represented by two bits, the two bits beinglocated after bit fields used for detecting a channel estimationcapability of the apparatus.
 16. A method for wirelessly communicatingbetween a first terminal having a first quantity of antennas and asecond terminal having a second quantity of antennas, the methodcomprising: transmitting a channel estimation capability of the firstterminal to the second terminal, wherein the channel estimationcapability is included in a transmit beamforming capability field andindicates a maximum number of space-time stream; and establishing awireless communication channel between the first terminal and the secondterminal based on the channel estimation capability; wherein the maximumnumber of space-time streams indicated by the channel estimationcapability is used to limit a number of columns of a channel matrix formulti-input multi-output (MIMO) communication, and wherein the signalsare transmitted through the wireless communication channel with bandlimiting by digital filter processing, a number of digital filterprocessing being equal to the first quantity of antennas.
 17. The methodof claim 16, further comprising: sending a sounding packet from thesecond terminal to the first terminal, wherein the sounding packetcomprises a training series for preparing a channel matrix for MIMOcommunication having the first quantity of rows and a third quantity ofcolumns, wherein the third quantity is equal to or less than the channelestimation capability.
 18. The method of claim 17, wherein the channelmatrix for MIMO communication is prepared by dividing the trainingseries into the third quantity of streams.
 19. The method of claim 17,further comprising: beamforming signals according to a transmissionweight matrix, thereby transmitting a data packet derived from thesignals via the wireless communication channel, wherein the transmissionweight matrix is determined according to the channel matrix for MIMOcommunication.
 20. The method of claim 16, wherein the channelestimation capability is represented by two bits, the two bits beinglocated after bit fields used for detecting a channel estimationcapability of the second terminal.
 21. The wireless communication systemof claim 1, wherein the transmit beamforming capability field isincluded in a HT capability element.
 22. The wireless communicationsystem of claim 21, wherein the HT capability element is present in amanagement frame.
 23. The wireless communication apparatus of claim 6,wherein the transmit beamforming capability field is included in a HTcapability element.
 24. The wireless communication apparatus of claim23, wherein the HT capability element is present in a management frame.25. The wireless communication apparatus of claim 11, wherein thetransmit beamforming capability field is included in a HT capabilityelement.
 26. The wireless communication apparatus of claim 25, whereinthe HT capability element is present in a management frame.
 27. Thewireless communication method of claim 16, wherein the transmitbeamforming capability field is included in a HT capability element. 28.The wireless communication method of claim 27, wherein the HT capabilityelement is present in a management frame.
 29. The wireless communicationsystem of claim 1, wherein the channel estimation capability isrepresented by two bits, the two bits being located adjacent to a bitfield including an information of maximum beamformer capability.
 30. Thewireless communication system of claim 1, wherein the transmitbeamforming capability field further includes both implicit beamformingcapability information and explicit beamforming capability information.31. The wireless communication apparatus of claim 6, wherein the channelestimation capability is represented by two bits, the two bits beinglocated adjacent to a bit field including an information of maximumbeamformer capability.
 32. The wireless communication apparatus of claim6, wherein the transmit beamforming capability field further includesboth implicit beamforming capability information and explicitbeamforming capability information.
 33. The wireless communicationapparatus of claim 11, wherein the channel estimation capability isrepresented by two bits, the two bits being located adjacent to a bitfield including an information of maximum beamformer capability.
 34. Thewireless communication apparatus of claim 11, wherein the transmitbeamforming capability field further includes both implicit beamformingcapability information and explicit beamforming capability information.35. The method of claim 16, wherein the channel estimation capability isrepresented by two bits, the two bits being located adjacent to a bitfield including an information of maximum beamformer capability.
 36. Themethod of claim 16, wherein the transmit beamforming capability fieldfurther includes both implicit beamforming capability information andexplicit beamforming capability information.
 37. A method for wirelesslytransmitting data from a transmitter having a first quantity of antennasto a receiver having a second quantity of antennas, the methodcomprising: notifying the receiver of a channel estimation capability ofthe transmitter, wherein the channel estimation capability is includedin a transmit beamforming capability field and indicates a maximumnumber of space-time stream; and transmitting signals from thetransmitter to the receiver based on the channel estimation capability,so as to establish a wireless communication channel between thetransmitter and the receiver; wherein the maximum number of space-timestreams indicated by the channel estimation capability is used to limita number of columns of a channel matrix for multi-input multi-output(MIMO) communication, and wherein the signals are transmitted throughthe wireless communication channel with band limiting by digital filterprocessing, a number of digital filter processing being equal to thefirst quantity of antennas.
 38. The method of claim 37, furthercomprising: receiving a sounding packet from the receiver, wherein thesounding packet comprises a training series for preparing the channelmatrix for MIMO communication having the first quantity of rows and athird quantity of columns, wherein the third quantity is equal to orless than the channel estimation capability.
 39. The method of claim 38,further comprising: dividing the training series into the third quantityof streams to prepare the channel matrix for MIMO communication.
 40. Themethod of claim 38, further comprising: beamforming signals according toa transmission weight matrix, thereby transmitting a data packet derivedfrom the signals via the wireless communication channel, wherein thetransmission weight matrix is determined according to the channel matrixfor MIMO communication.
 41. The method of claim 37, wherein the channelestimation capability is represented by two bits, the two bits beinglocated after bit fields used for detecting a channel estimationcapability of the receiver.
 42. The method of claim 37, wherein thechannel estimation capability is represented by two bits, the two bitsbeing located adjacent to a bit field including an information ofmaximum beamformer capability.
 43. The method of claim 37, wherein thetransmit beamforming capability field further includes both implicitbeamforming capability information and explicit beamforming capabilityinformation.
 44. A method for wirelessly receiving data by a receiverhaving a first quantity of antennas from an initiator having a secondquantity of antennas, the method comprising: receiving notification of achannel estimation capability of the initiator, wherein the channelestimation capability is included in a transmit beamforming capabilityfield and indicates a maximum number of space-time stream; and receivingsignals from the initiator based on the channel estimation capability,so as to establish a wireless communication channel between the receiverand the initiator; wherein the maximum number of space-time streamsindicated by the channel estimation capability is used to limit a numberof columns of a channel matrix for multi-input multi-output (MIMO)communication, and wherein the signals are received with band limitingby digital filter processing, a number of digital filter processingbeing equal to the first quantity of antennas.
 45. The method of claim44, further comprising: transmitting a sounding packet to the initiator,wherein the sounding packet comprises a training series for preparingthe channel matrix for MIMO communication having the second quantity ofrows and a third quantity of columns, wherein the third quantity isequal to or less than the channel estimation capability.
 46. The methodof claim 45, wherein the channel matrix for MIMO communication isprepared by dividing the training series into the third quantity ofstreams.
 47. The method of claim 45, further comprising: receivingsignals beamformed by the initiator according to a transmission weightmatrix, thereby receiving a data packet derived from the signals via thewireless communication channel, wherein the transmission weight matrixis determined according to the channel matrix for MIMO communication.48. The method of claim 44, wherein the channel estimation capability isrepresented by two bits, the two bits being located after bit fieldsused for detecting a channel estimation capability of the receiver. 49.The method of claim 44, wherein the channel estimation capability isrepresented by two bits, the two bits being located adjacent to a bitfield including an information of maximum beamformer capability.
 50. Themethod of claim 44, wherein the transmit beamforming capability fieldfurther includes both implicit beamforming capability information andexplicit beamforming capability information.
 51. A first communicationdevice for wireless communication, the communication device comprising:processing circuitry configured to: notify a second communication deviceof a channel estimation capability indicating a maximum number of spacetime streams, the channel estimation capability being included in apredetermined management frame; and control transmitting signals to thesecond communication device based at least in part on informationincluded in the predetermined management frame; wherein the signals aretransmitted with band limiting by digital filter processing, a number ofdigital filter processing being equal to a number of antennas used fortransmission.
 52. The first communication device of claim 51, whereinthe predetermined management frame includes HT capabilities elementdefined in wireless network standard IEEE802.11.
 53. A secondcommunication device for wireless communication, the communicationdevice comprising: processing circuitry configured to: receivenotification from a first communication device of a channel estimationcapability indicating a maximum number of space time streams, thechannel estimation capability being included in a predeterminedmanagement frame; and control receiving signals from the firstcommunication device based at least in part on information included inthe predetermined management frame; wherein the signals are transmittedwith band limiting by digital filter processing, a number of digitalfilter processing being equal to a number of antennas used forreception.
 54. The second communication device of claim 53, wherein thepredetermined management frame includes HT capabilities element definedin wireless network standard IEEE802.11.